Built-in leakage radiation detecting device for a microwave oven

ABSTRACT

An electronic microwave oven has incorporated therewithin a leakage radiation detecting device for sensing and providing an indication of the presence of radiation escaping from the oven cavity past the door seal. The device, located within the oven door, includes a pickup loop, sensitive to the magnetic field of the microwave radiation, partially projecting into the channel formed by the inner surface of the door and the door frame external to the seal gasket. A transmission line, located coaxially within a cylindrical waveguide communicating with the leakage radiation, is resonant to a quarter wavelength of the operating microwave frequency and operatively couples the pickup loop with a neon lamp. When radiation above a predetermined level escapes from the oven cavity, a voltage is induced within the pickup loop which, by virtue of the resonance effect of the transmission line, is sufficient to light the neon lamp.

United StatesPatent [1 1 Fitzmayer BUILT-IN LEAKAGE RADIATION DETECTINGDEVICE FOR A MICROWAVE OVEN [75] Inventor:

[73] Assignee: General Electric Company,

Louisville, Ky.

[22] Filed: Aug. 7, 1972 [21] Appl. No.: 278,596

Louis B. Fitz'mayer, Louisville, Ky.

[52] U.S. Cl. 219/1055, 324/122 511' rm, Cl. H05b 9/06 581 Field ofSearch ..219/1o.ss;324/9s,

[ 1 3,748,424 July 24, 1973 Primary Examiner-J. V. Truhe AssistantExaminer-Hugh D. Jaeger Attorney-Richard L. Caslin et al.

[57] ABSTRACT An electronic microwave oven has incorporated therewithina leakage radiation detecting device for sensing and providing anindication of the presence of radiation escaping from the oven cavitypast the door seal. The device, located within the oven door, includes apickup loop, sensitive to the magnetic field of the microwave radiation,partially projecting into the channel formed by the inner surface of thedoor and the door frame external to the seal gasket. A transmissionline, located coaxially within a cylindrical waveguide communicatingwith the leakage radiation,-is resonant to'a quarter wavelength of theoperating microwave frequency and operatively couples the pickup loopwith a neon lamp. When radiation above a predetermined level escapesfrom the oven cavity, a voltage is induced within the pickup loop which,by virtue of the resonance effect of the transmission line, issufficient to light the neon lamp.

5 Claims, 5 Drawing Figures BUILT-IN LEAKAGE RADIATION DETECTING DEVICEFOR A MICROWAVE OVEN BACKGROUND OF THE INVENTION In the accompanyingdrawings: FIG. 1 is a perspective view of a typical domestic The presentinvention relates to a device for sensing freestanding range includingan electronic microwave and indicating the presence of microwaveradiation and in particular, a device built into a domestic microwaveoven for sensing and indicating the presence of microwave radiationleaking from the oven cavity past the oven door seal.

Manufacturers of electronic microwave ovens take extreme care in thedesign of the oven to insure the safety of the user against leakage ofmicrowave radiation, particularly around the oven door seal. However,despite the precautions taken by the manufacturer, it is sometimespossible for an oven to become leaky during its lifetime due to aging ofthe door seal gasket or due to accumulation of spilled food particles onthe door seal. For the safety of the user, the Department of Health,Education and Welfare/Bureau of Radiological Health regulations specifythat the maximum allowable leakage from any oven surface, other thanin-factory measurements, be 5 milliwatts per square centimeter measured5 centimeters from any oven surface.

A reliable device is needed, therefore, to sense and give to theoperator an indication of possible leakage radiation at such a lowlevel. One device is described in US. Pat. No. 3,436,508-Fritz wherein apair of conductors are located on the oven cabinet just beyond the outeredge of the door opening and looped therearound and which are connectedto an alarm means for announcing the presence of leakage radiation. Sucha device, however, has certain disadvantages, the most important ofwhich is lack of sensitivity. With conductor wires mounted in such afashion, there is very little possibility of their being responsive tothe electric field of the radiation since they generally would not beparallel with this electric field. Neither would they be responsive tothe magnetic field of the radiation, the net effect upon the wires inthis case being normally close to zero.

It is therefore the principle object of this invention to provide adevice built into a domestic microwave oven for sensing and indicatingthe presence of microwave radiation leaking from the oven cavity pastthe door seal, a device that is sensitive to such small leaks asmentioned above and is simple and trouble free as well as reliable andlow cost.

SUMMARY OF THE INVENTION lnaccordance with the present invention, aleakage radiation detecting device is designed to be incorporated withina microwave oven of the type having a box-like oven liner and an accessdoor defining an oven cooking cavity. Microwave radiation seal means areprovided between the door and the oven liner. Included is sensor meanspartially projecting into the space formed between the access door andthe oven frame external to the seal means, the sensor means arranged soas to communicate with the magnetic field of the microwave leakageradiation. transmission means, resonant to the operating microwavefrequency and located coaxially within a waveguide communicating withthe leakage radiation, is coupled with the sensor means. Indicatormeans, responsive to the output of the resonant transmission means, iscoupled therewith.

oven;

FIG. 2 is an enlarged fragmentary view, partly in section, taken alongthe line 2-2 of FIG. 1, of a portion of an electronic microwave ovenincorporating a preferred embodiment of the present invention;

FIG. 3 is a further enlarged sectional view of a preferred embodiment ofthe present invention shown in FIG. 2;

FIG. 4 is an end elevation of the view in FIG. 3; and

FIG. 5 is a fragmentary view of the-device of FIG. 2 showing analternate embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 of thedrawing, there is illustrated a household electric range 10, the range10 including an electronic microwave oven 11 embodying the features ofthe present invention.

Referring now to FIG. 2, a substantially box-like metallic oven liner20, a portion of which is shown, in conjunction with a metallic accessdoor 25, shown in the closed or vertical position, defines a microwaveoven cooking cavity 26 therein. Liner 20 includes a rear wall (notshown), a top wall 28 (partially shown), a bottom wall (not shown), anda pair of side walls 30, one of which is shown. Forward edges of the topwall 28, the bottom wall and the side wall 30 define an opening into thecooking cavity 26, there being a continuous outwardly facing flange 31which is bent back upon itself on its rear surface as shown. The frontdoor frame 32 has an opening therein slightly greater than the areadefined by the outer edges of the flange 31 and has a rearwardlydirected flange 33 which is suitably secured to the several walls of theoven liner 20 so as to interconnect the front door frame 32 and the ovenliner 20.

Front opening access door 25 is mounted on the front of the oven 11 byhinge structure (not shown) and is movable about the lower edge thereofbetween a substantially horizontal open position and a substantiallyvertical closed position with respect to the open front of the ovencavity 26. Access door 25 more particularly includes a metal outer sheet34, a metal inner sheet 35 spaced well inwardly of the outer sheet 34,and a metal intermediate sheet 36 disposed between the outer sheet 34andthe inner sheet 35. The outer sheet 34 carries an inturned flange 37around the periphery thereof, flange 37 extending inwardly toward and toa point spaced a short distance from the front door frame 32 when theaccess door 25 is in its sheet 36 position. In-' tennediate sheet 36 hasa forwardly directed flange 38 that carries an outwardly directed flange39 extending therearound, flange 39 being suitably secured to flange 37.As illustrated, flange 38 forming a portion of the inner surface of thedoor 25 is disposed substantially parallel to and spaced a shortdistance from a portion of the flange 33 so as to define a channel likespace such as continuous channel 40. Inner sheet 35 has a forwardlydirected flange 41 around the periphery thereof extending toward theintermediate sheet 36 and is suitably secured thereto.

Microwave radiation seal means, such as metallic radiation seal gasket45, is fixedly attached to the intermediate sheet 36 of door 25 andprovides a seal and an electrical connection between the outer surfaceof the oven liner 20 and the inner surface of the door 25 andspecifically, between the outwardly facing flange 31 on the oven liner20 and the intermediate sheet 36 of the access door 25. Gasket 45 may beof the type described in U.S. Pat. No. 3,459,92l-Fussell and assigned tothe same assignee as the present invention.

In accordance with the present invention a leakage radiation detectingdevice, such as that denoted generally as 50 in FIG. 2, is designed tobe incorporated within microwave oven 11. Device 50 includes sensormeans such as pickup loop 51 partially projecting into channel 40 so asto communicate with the magnetic field of the microwave leakageradiation. As can be more clearly seen in FIG. 3, pickup loop 51 has oneleg coupled to a common terminal as 52. Pickup loop 51 has its other legconnected to indicator means such as neon lamp 53 through transmissionmeans such as transmission line 54 located coaxially within a generallycylindrical waveguide 55. Spacing and isolation of transmission line 54from waveguide 55 is accomplished by insulator 56, more clearly shown inFIG. 4. Waveguide 55 communicates with the leakage radiation throughchannel 40 and transmission line 54 is a frequency resonant conductor,resonant to the operating microwave frequency of microwave oven 11.

Neon lamp 53 is mounted within the door structure so as to be visiblefrom the outside of the oven 11 and has its other terminal seriallycoupled to common terminal 52 through a resistance such as currentlimiting resistor 57, thereby completing the electric circuit withpickup loop 51. There may be a capacitive or direct connection betweenresistor 57 and common terminal Radiation leaking from oven cavity 26past seal gasket 45 at any location thereof, will find its way into thespace between the door 25 and the oven frame and especially withinchannel 40. Channel 40 in this case will be acting as a waveguide and astanding wave will be set up in the channel 40 around the entireperiphery thereof. In the operation of the device of the presentinvention, since pickup loop 51 is sensitive to the magnetic field ofthe microwave radiation and since it is arranged to communicatetherewith, a changing magnetic field will cause a voltage to be inducedwithin pickup loop 51. This voltage will likewise appear in transmissionline 54 and since waveguide 55 communicates with channel 40, and furthersince some of the leakage radiation will also appear in waveguide 55,transmission line 54 will become resonant and the voltage appearingtherein will be amplified causing neon lamp 53 to become activated.

An alternate embodiment is shown in FIG. wherein a probe 60 is sensitiveto the electric field of the microwave radiation. Since probe 60communicates with the electric field, a voltage will be generated withinthe probe which will be caused to be amplified within transmission line54 further causing neon lamp 53 to become activated.

In laboratory tests, waveguide 55 was made of stainless steel tubingwith the inside diameter represented by the letter d in FIG. 3, chosento be three-quarters of an inch. Resonant transmission line 54 was madeof 1/8 inch diameter stainless steel tubing joined with one of theelectrodes of neon lamp 53 with the length represented by the letter Iin FIG. 3 chosen to be resonant to a quarter wavelength of the operatingmicrowave frequency, which in this case was 915 MHz, thereby giving I alength of approximately 3 inches. Insulator 56 was made of Teflon. Itwas found that if probe 60 projected into the electric field, as shownin FIG. 5 by the distance x, and if this distance were 1/8 inch, leakageradiation of energy levels as low as five milliwatts per squarecentimeter measured 5 centimeters from any oven surface, could bedetected.

For purposes of calibrating device 50 to become activated only atradiation levels above a predetermined level, the length x of probe 60in FIG. 5 need only be varied, i.e., for more sensitivity, the lengthwill be increased. For a sensor of the type of pickup loop 51, which issensitive to the changing magnetic field of the radiation, one needchange the area a shown in FIG. 3 to vary its sensitivity, i.e., formore sensitivity, the area will be increased.

For further sensitivity a pair of such devices 50 could be mountedwithin door 25 as shown in FIG. 1 at alternate corners of the door.Further, the lens 62 is added to protect the neon lamp as well as toenhance the appearance of the structure and to make the lighted lampmore clearly visible.

It should be apparent to those skilled in the art that the embodimentsdescribed heretofore are considered to be the presently preferred formof this invention. In accordance with the Patent Statutes, changes maybe made in the disclosed apparatus and the manner in which it is usedwithout actually departing from the true spirit and scope of thisinvention.

What is claimed is: 1. In a microwave oven of the type having a box-likeoven liner and an access door defining an oven cooking cavity, andmicrowave radiation seal means between the door and oven liner, aleakage radiation detecting device comprising:

sensor means partially projecting into the space formed between theaccess door and the oven frame external to the seal means, the sensormeans being sensitive to and arranged so as to communicate with themagnetic field of the microwave leakage radiation;

transmission line means, resonant to the operating microwave frequency,coupled with the sensor means and located coaxially within a waveguidecommunicating with the leakage radiation; and

indicator means coupled with and responsive to the output of theresonant transmission means to become activated;

whereby, in the presence of radiation above a predetermined levelleaking from the oven cavity past the seal means, a voltage induced inthe sensor means will be amplified in the resonant transmission linemeans sufiiciently to activate the indicator means to provide anindicium of the presence of such leakage radiation.

2. The device of claim 1 wherein the sensor means is arranged so as tocommunicate with the electric field of the microwave leakage radiationand is sensitive thereto such that a voltage will be generated in thesensor means to be amplified in the resonant transmission line means.

3. In a microwave oven having a metallic box-like oven liner and ametallic access door hingedly attached to the oven at the bottom thereofforming an oven cooking cavity, a front frame surrounding the ovenliner, meansfor supplying microwave energy to the oven cavity, aconductive radiation seal gasket attached to the inner surface of thedoor and positioned 5 to contact the liner flange around the entireperiphery thereof when the door is closed, and a continuous channel-likespace defined by the inner surface of the door and the front frameexternal to the seal gasket, a leakage radiation detecting devicecomprising:

ranged so as to communicate with and is sensitive to the electric fieldof the microwave leakage radiation.

5. In a microwave oven having a metallic box-like oven liner and ametallic access door hingedly attached to the oven at the bottom thereofforming an oven cooking cavity, a front frame surrounding the ovenliner, means for supplying microwave energy to the oven cavity, ametallic radiation seal gasket attached to the inner surface of the doorand positioned to contact the liner flange around the entire peripherythereof when the door is closed, and a continuous channel defined by theinner surface of the door and the front frame external to the sealgasket, a leakage radiation detecting device located within the doorcomprising:

an inductive pickup loop partially projecting into the channel andarranged so as to communicate with the magnetic field of the microwaveleakage radiation;

a conductive transmission line resonant to a quarter wavelength of theoperating microwave frequency, operatively coupled with the pickup loopand located coaxially within and isolated from a cylindrical waveguideand communicating with the channel and thereby the leakage radiation;and

a neon lamp having one of its terminals coupled with and responsive tothe output of the resonant transmission line to become activated, andthe other terminal connected with a common ground and thereby with thepickup loop to complete the circuit.

1. In a microwave oven of the type having a box-like oven liner and anaccess door defining an oven cooking cavity, and microwave radiationseal means between the door and oven liner, a leakage radiationdetecting device comprising: sensor means partially projecting into thespace formed between the access door and the oven frame external to theseal means, the sensor means being sensitive to and arranged so as tocommunicate with the magnetic field of the microwave leakage radiation;transmission line means, resonant to the operating microwave frequency,coupled with the sensor means and located coaxially within a waveguidecommunicating with the leakage radiation; and indicator means coupledwith and responsive to the output of the resonant transmission means tobecome activated; whereby, in the presence of radiation above apredetermined level leaking from the oven cavity past the seal means, avoltage induced in the sensor means will be amplified in the resonanttransmission line means sufficiently to activate the indicator means toprovide an indicium of the presence of such leakage radiation.
 2. Thedevice of claim 1 wherein the sensor means is arranged so as tocommunicate with the electric field of the microwave leakage radiationand is sensitive thereto such that a voltage will be generated in thesensor means to be amplified in the resonant transmission line means. 3.In a microwave oven having a metallic box-like oven liner and a metallicaccess door hingedly attached to the oven at the bottom thereof formingan oven cooking cavity, a front frame surrounding the oven liner, meansfor supplying microwave energy to the oven cavity, a conductiveradiation seal gasket attached to the inner surface of the door andpositioned to contact the liner flange around the entire peripherythereof when the door is closed, and a continuous channel-like spacedefined by the inner surface of the door and the front frame external tothe seal gasket, a leakage radiation detecting device comprising: asensor partially projecting into the channel-like space and sensitive toand arranged so as to communicate with the magnetic field of themicrowave leakage radiation; a transmission line, resonant to theoperating microwave frequency, operatively coupled with the sensor andlocated coaxially within and isolated from a waveguide communicatingwith the channel-like space and thereby, the leakage radiation; and anindicator coupled with and responsive to the output of the resonanttransmission line to become activated.
 4. The device of claim 3 whereinthe sensor is arranged so as to communicate with and is sensitive to theelectric field of the microwave leakage radiation.
 5. In a microwaveoven haviNg a metallic box-like oven liner and a metallic access doorhingedly attached to the oven at the bottom thereof forming an ovencooking cavity, a front frame surrounding the oven liner, means forsupplying microwave energy to the oven cavity, a metallic radiation sealgasket attached to the inner surface of the door and positioned tocontact the liner flange around the entire periphery thereof when thedoor is closed, and a continuous channel defined by the inner surface ofthe door and the front frame external to the seal gasket, a leakageradiation detecting device located within the door comprising: aninductive pickup loop partially projecting into the channel and arrangedso as to communicate with the magnetic field of the microwave leakageradiation; a conductive transmission line resonant to a quarterwavelength of the operating microwave frequency, operatively coupledwith the pickup loop and located coaxially within and isolated from acylindrical waveguide and communicating with the channel and thereby theleakage radiation; and a neon lamp having one of its terminals coupledwith and responsive to the output of the resonant transmission line tobecome activated, and the other terminal connected with a common groundand thereby with the pickup loop to complete the circuit.